During the differentiating of velocity values v present in digital form or of values corresponding to them, a minimum permissible differential velocity .DELTA.v versus the value at the beginning of the assigned time .DELTA.t must be exceeded to generate deceleration -b and acceleration +b control signals, during an assigned time .DELTA.t.
When the starting mode of the vehicle wheel exhibits a wrong tendency upon removal of the b signals, this is an indication that the chosen assigned time was too long. If, for example, two like -b signals appear before an unlike +b signal, it appears this would indicate that the chosen assigned time was too short. Further, if the +b signal reappears before a -b signal appears, this is an indication that the switching time of the +b signal was too short or the air release phase was too short. In each of these situations, there is a risk that the reference velocity or wheel velocity deviates too much from the vehicle velocity and proper control of the brake pressure is no longer possible. The manner of deriving and forming the reference velocity is shown and described in U.S. Pat. No. 4,113,322 which is assigned to the assignee of the present application.
A known method for generating digital acceleration and deceleration signals is shown and described in German Pat. No. 2,342,358, in which an assigned time or measuring period of a given duration is provided and in which corresponding signals are generated when given difference values are exceeded. The patented process is based on the selection of relatively long assigned times and large difference values for filtering purposes so that disturbing signals cannot simulate the generation of deceleration control and acceleration control signals for a short time. In order to improve the dropout behavior, a change in length of the assigned time and a shift of the difference value to a smaller predetermined value it provided when the acceleration or deceleration control signals appear within the assigned time. At the end of each expired assigned time, the measurement of the preceding velocity is stored and compared with the next value measured. As soon as a larger difference value is determined, the current assigned time is interrupted and restarted. The arrangement provided to carry out the process included a time delay element having a time constant corresponding to the duration of the period and which was shifted by a switching signal, namely a -b control signal. The control signal was generated by a comparator circuit when the measured value exhibits a predetermined deviation from the reference value which was stored in the memory bank. In order to prevent the switching signals from terminating too early, a relatively long time is provided for the measuring period to compensate for external disturbances. These disturbances in the wheel behavior can be caused by dynamic axle shifting as well as by unfavorable road conditions. In the previous operations, the most unfavorable was anticipated which means that a delayed decrease is employed even if the unfavorable condition programmed into the system does not occur.